1. Field of the Invention
The present invention relates to detectors for detecting an oriented physical variable, including a sensor having the shape of a flat chip and sensitive to the physical variable along a predetermined axis, and a board bearing electronic components, that are accommodated in a casing.
It is known that the detection of an oriented physical variable, such as an acceleration or a magnetic field, capable of being represented by a mathematic vector, depends on the measurement of its intensity (module of the vector) and on the determination of its orientation (coordinates of the vector).
2. Discussion of the Related Art
In the field of the detectors of oriented physical variables, recent improvements in etching of crystalline materials have made it possible to fabricate miniaturized chip-shaped sensors, such as, for example, acceleration microsensors or magnetic field microsensors, that advantageously replace the conventional large-size electromechanic or magnetomechanic systems.
However, because of technological constraints, such sensors generally have only a single axis of sensitivity to a physical variable; moreover, the orientation of such axis with respect to the plane of the chip is imposed by the manufacturing technology.
FIG. 1 shows a conventional detector 15 using a miniaturized sensor 1 that can be, for example, an accelerometer or a magnetometer. The chip-shaped sensor 1 is associated with an electronic control circuit 2. The sensor 1 and the electronic circuit 2 are assembled in a single electronic casing 3, represented without a lid to show the various components. The casing 3 includes lateral walls 4, fastening ears 5, and connecting pins 6, for connecting circuit 2 to external components. Circuit 2 has also to be miniaturized and thus can be, for example, a hybrid circuit formed on a ceramic substrate 7 that includes conductive paths 8 for the interconnection of electronic components 9 mounted on substrate 7. The sensor 1 is provided with plated pads 10, connected to metal Lines 8 of the circuit by means of aluminum or gold threads 11, that are welded by ultrasonic bonding.
As indicated above, the sensor 1 is sensitive along a single axis, oriented in a predetermined direction, imposed by the manufacturing technology. In the case of an acceleration sensor, according to the manufacturing technology, the axis of sensitivity 16 can be a transverse axis, perpendicular to the plane of the chip, or a lateral axis 17, parallel to the plane of the chip. In the case of a magnetic field sensor, the axis of sensitivity is generally a transverse axis 16. These various orientations are illustrated in FIG. 1 by axes 16 and 17.
Conventionally, as shown in FIG. 1, sensor 1 is fastened flatwise in casing 3 and disposed in parallel with substrate 7. This positioning is imposed by the ultrasonic bonding technique; such technique requires that the plane of pads 10 be parallel with the plane of metal lines 8 of the substrate. It should be noted that ultrasonic bonding is carried out by means of a tool vibrating at a high frequency (typically 60 KHz) and including a tapered end that abuts against the wiring thread, facing the surface to be welded; the vibration energy transmitted to the thread thus causes, due to friction and heating, welding of the thread. Since the vibrating tool has a fixed orientation, the various points to be connected must be disposed on a same plane, or at least on planes that are substantially parallel. Another equivalent known method consists in mounting the sensor 1 directly on the casing bottom, close to substrate 7.
So, depending on the type of sensor 1 mounted in casing 3, the axis of sensitivity of sensor 15 is therefore oriented perpendicularly to the fixation plane of casing 3, or parallel with this plane, as shown in FIG. 1.
Indeed, the necessity of orienting sensor 1 in a predetermined direction (the direction in which the physical variable is to be measured) may require enlarging casing 3 in a way that raises size problems. This drawback is particularly crucial when the detector is to be mounted in a small-size equipment.
This drawback is also encountered in the case of a system having several axes of detection, for the detection of a physical variable whose orientation and intensity are unknown. To illustrate this last case, FIG. 2 shows a 3-axis accelerometric system 20 fabricated with three accelerometers 15, 15', 15", each including an acceleration sensor, 1, 1', 1", having a transverse axis of sensitivity 16, 16', 16". In this figure, the same reference numerals designate same elements as in FIG. 1. For the fabrication of the 3-axis system 20, the three casings 3, 3', 3" of accelerometers 15, 15', 15" had to be mounted on the respective surfaces 22, 23, 24 of a mechanic support 21 forming a reference trihedron. As can be seen, such an arrangement is cumbersome: the length of each casing 3, 3', 3" being typically approximately 10 centimeters, the volume occupied by the 3-axis system 20 is at least approximately 1 dm.sup.3, which is a large volume in an electronic equipment.